COMMISSION 27 OF THE I.A.U. INFORMATION BULLETIN ON VARIABLE STARS Number 2963 Konkoly Observatory Budapest 5 December 1986 HU ISSN 0374 - 0676 ON THE PERIOD VARIATION OF YZ Boo YZ Boo is one of the few high amplitude Delta Scuti stars. The investigation of their period changes may give some information about their evolutionary state. Therefore we carried out new photometry of YZ Boo with the aim of investigating its period variation. The period changes of YZ Boo was first studied by Szeidl and Mahdy (1981). They suspected that a slight continuous increase of the period had taken place: Max.hel. = J.D. 2442146.3543 + 0d.104091580*E + 5d.3x10E-13*E^2 Jiang (1986) reexamined all the observations published up till 1984 and made a thorough investigation of the period changes of YZ Boo and found the new ephemeris with a quadratic term: Max.hel. = J.D. 2442146.35481 + 0d.1040915791*E + 4d.3x10E-13*E^2 Peniche et al. (1985) carried out new accurate photometry of YZ Boo and made a detailed study of the period variation of the star independently of Jiang. Peniche et al. (1985) took the old poorly determined visual observations of Tsessevich (1958) into account, as well, and came to the conclusion that the period was basically constant: Max.hel. = J.D. 2442146.35481 + 0d.1040915680*E + 1d.12x10E-13*E^2 where the uncertainty in the quadratic coefficient is of the order of the derived value. In order to study of the period changes of YZ Boo and to determine a more accurate ephemeris we observed the star from 24 June until 12 July 1986 on 13 nights. The observations were carried out by the 74 inch telescope at Kottamia Observatory, Egypt. Standard U, B and V filters with EMI 9558B tube were used. The observed times of light maxima are given in Table I. In columns 2 and 3 the cycle numbers and the O-C_lin values are given calculated according to the linear elements of Jiang (1986): C_lin = J.D. 2442146.35531 + 0d.1040915678*E Table I Observed light maxima of YZ Boo J.D.hel. E O-C_lin O-C_quad 2446606.3697 42847 +0d0030 +0d0023 612.3000 42904 +0.0001 -0.0006 612.4040 42905 0.0000 -0.0007 613.3399 42914 -0.0010 -0.0016 614.2773 42923 -0.0004 -0.0010 614.3825 42924 +0.0007 +0.0001 615.3193 42933 +0.0007 +0.0001 617.2972 42952 +0.0009 +0.0002 618.3361 42962 -0.0011 -0.0018 619.2745 42971 +0.0004 -0.0002 620.3170 42981 +0.0020 +0.0014 621.3569 42991 +0.0010 +0.0003 622.2936 43000 +0.0009 +0.0002 623.3338 43010 +0.0002 -0.0005 624.2713 43019 +0.0008 +0.0002 624.3770 43020 +0.0024 +0.0018 The list of light maxima of YZ Boo given in the paper of Peniche et al. (1985) were supplemented with Jiang's (1986) and our observed maxima and then a least-squares solution was carried out. The photoelectric maxima obtained in the last 30 years were only taken into account. As the weights assigned to the times of light maxima may sometimes be a matter of subjective judgement, equal weights were given to each time of light maxima in our calculations. The following new quadratic equation has been obtained: C_quad = Max.hel. = J.D. 2442146.35479 + 0d.1040915805*E + 3d.43x10E-13*E^2 +-26 +-41 +-1.47 This formula has been valid since 1955, when the first photoelectric observations were carried out by Eggen (1955). Our observations show definite light curve variations. Both the shape and the amplitude of the light curve are subjected to changes from cycle to cycle. An overtone pulsation is probably excited beside the fundamental mode. In a forthcoming paper we will carry out a detailed period analysis of this star. MOUNIR A. HAMDY HAMID A. MAHDY MOHAMMED A. SOLIMAN Helwan Observatory Egypt References: Eggen, O.J., 1955, Publ. Astr. Soc. Pacific 67.354 [BIBCODE 1955PASP...67..354E ] Jiang, Shi-yang, 1986, Chinese Astron, Astrophys. 10.115 [BIBCODE 1986ChA&A..10..115J ] Peniche, R., Gonzalez, S.F. and Pena, J.H., 1985, Publ. Astr. Soc. Pacific 97.1172 [BIBCODE 1985PASP...97.1172P ] Szeidl, B. and Mahdy, H.A., 1981, Comm. Konkoly Observatory, Budapest, No. 75 [BIBCODE 1981CoKon..75....1S ] [CoKon No. 75] Tsessevich, V.P., 1958, Astron. Circ, No. 118.24